Mr Roberts, on the national executive of both the ATL and National Union of Teachers, promised to revive debate about whether the three big unions should merge.

Mr Roberts decided to stand against the leadership’s preferred candidate, Dr Mary Bousted, because ATL members “deserve better than a ‘choice’ of one”.

One of his key priorities would be to overturn the union’s policy and oppose plans to let assistants take charge of classes. His decision to fight the plan comes at a difficult time for the ATL, which with other unions is expected to agree a deal with Government over workload. His candidature will be seen as a test of support for a “super” union.

Merger plans have stalled since the National Association of Schoolmasters Union of Women Teachers rejected their general secretary Eamonn O’Kane‘s plans for a single classroom union.

Last March a TES survey found that a majority of teachers backed a single union, although ATL members were the most sceptical with just 52 per cent in favour.

Gerald Imison, ATL deputy general secretary is also believed to be considering a second bid for the post after narrowly missing out on the leadership’s endorsement.

Both Mr Roberts and Mr Imison need the support of six branches before the February 14 deadline. Mr Roberts intends to contrast his experience of union and teaching work with that of Dr Mary Bousted, an outsider selected by the ATL national executive to replace Peter Smith, who retired on health grounds.Dr Bousted, head of Kingston University school of education, has never been a member of the ATL and has no union experience, although she is a former member of the NUT.

A co-founder of Professional Unity 2000, launched in 1996, Mr Roberts is a member of all three big classroom unions and teaches at Copland community school, in Wembley, north London.